Optical correlators use the Fourier transform property of lenses to compare optical signals, such as to compare two images (or other two-dimensional signal) and correlating features in one image with features in the other. Implementing such a calculation in silicon (i.e. electronic computing) becomes increasingly expensive as the resolution increases. By contrast, the scaling of the optical correlator is more favorable, although for low resolution state-of-the-art silicon remains superior.
However, in many instances, optical correlators are composed of large, tabletop optical components, which are hand-assembled and aligned, expensive, sensitive to vibration, etc. The effective compute power per unit volume is quite low.